First, You need to give yourself membership the Discovery Management role and add the Mailbox Import Export role to the Discovery Management role.
The content will be placed in a top level folder not merged with the main folder tree, but once in there the content can be dragged out to the main tree as required.
Run Powershell as admin:
Search-Mailbox -Identity "MAILBOX1" -TargetMailbox "MAILBOX2" -TargetFolder "Moved From Old Mailbox" -LogLevel Full
*Once done you can run the above against any mailboxes.
**Alternatively, you can use a search process to copy the entire content from one mailbox to a folder in a target mailbox.